Long description

The figure shows two parallel horiontal lines. The upper one is labelled q1 and the lower one is labelled q2. In the left part of the upper line is a square box with a capital letter H in it. Further along the upper line is a large dot. A vertical line leads from this dot to the lower line where it joins a large circle with a large plus sign in it. to the right of this on the lower line is another large circle with a large plus sign in it.